1. Industrial Application Field
The present invention relates to a 3-position rotational actuator capable of controlling three positions, i.e., one stable position of a rotor due to a field produced by one pair of four poles of a field pole and two stable positions given by stopping rotation immediately before the stable points due to positive and negative fields produced by another pair of the four poles thereof.
2. Prior Technique
Conventionally, a device for digitally controlling the rotational position is generally known as a stepping motor which is actually used in various applications. However, although being a simple system for attaining three positions, the stepping motor is required to have 6-pole field windings, thereby resulting in being unsuitable for use in areas requiring size-reduction and weight-reduction.
Thus, a 3-position rotational actuator has been proposed to allow control of three positions by use of only a 4-pole field pole. FIG. 8 is a schematic illustration for describing the operation principal of the 3-position rotational actuator. In FIG. 8, in response to operation of a switch S1, one pair of poles .phi.A and .phi.B of four field poles are spaced so as to oppose each other, and are excited to be of opposite polarities. In this state, a rotor R made up of a cylindrical permanent magnet is stable with its magnetic axis being coincident with the line of magnetic flux developed by the field poles .phi.1A and .phi.1B. After release of the switch S1, since rotor R is made of a permanent magnet, the rotor R is kept stable due to generation of a detention torque (position B in the Figure).
In response to operation of a switch S2, another pair of poles .phi.2A and .phi.2B are excited (direction of the field at this time is positive) and the rotor R receives a rotational force tending to rotate it counterclockwise from the stable position indicated by B until the magnetic axis of the rotor R is coincident with the line of magnetic flux developed by the poles .phi.2A and .phi.2B. However, a rotation-limiting member is provided with respect to the rotor R so as not to cause the magnetic axis to be coincident with the line of magnetic flux due to the counterclockwise rotation of rotor R, whereby the rotor R stops when the magnetic axis is rotated to a position indicated by A in the Figure. Similarly, in response to turning on a switch S3, although receiving a clockwise rotational force, the rotor R stops due to a rotation limiting member after rotated up to a position indicated by C in the Figure.
Irrespective of such a simple structure, a 3- position rotational actuator can be realized which is controllable to take the three positions (stable points) indicated by A, B, C. Because it is small in size and light in weight, it has been used as an actuator of motor vehicles, for example. Problems to be Resolved by the Invention
However, such a 3-position rotational actuator is unsatisfactory, and has the following problems.
Characteristics necessary for stepwise position control, not limited to a 3-position rotational actuator, are that the holding torque at the stable point is great and the drive torque is great on shifting from one stable point to another stable point. Even in the case of the conventional 3-position rotational actuator, although it is possible to increase the magnetic force of the permanent magnet of the rotor R and further increase the holding torque (detention torque) and drive torque by enlarging the size of the field pole, this results in new problems such as of the apparatus which limits the application and increases the cost. For resolving these problems, the present inventors have made the following experiment for improving the state of the magnetic flux by varying the sizes of the magnetic pole pieces of the field pole. FIG. 9 are descriptive diagrams showing the relationship between the sizes of the magnetic poles of the field pole of a 3-position rotational actuator and the detention torque, or the drive torque. As shown in FIG. 9A, the size of each member of one pair of magnetic pole pieces can be expressed by an angle (which will be referred to as core stator angle) .phi.A of the rotor R with respect to the center of rotation. FIG. 9B depicts characteristic curves showing the variation of the B-position detention torque in accordance with variation of the core stator angle .theta.A and the variation of the drive torque on shifting from position B to position A or C. Here, the size of each of the other pair of magnetic pole pieces is kept constant (.theta.B=78.degree.). As obvious from FIG. 9b, when the core stator angle .theta.A is small, the detention torque becomes great and the drive torque becomes small, and on the other hand, when when the core stator angle .theta.A is great, the drive torque becomes great and the detention torque becomes small. This may result from that fact that, when the core stator angle .theta.A is small, the magnetic flux is concentrated at the center of the field pole, the magnetic flux from the rotor R for generating the detent torque with respect to the rotor R is centered at the centers of field poles .phi.1A and .phi.1B so as to heighten the magnetic flux density, and the pole-spacing distance between the magnetic poles due to the field poles .phi.2A and .phi.2B for driving the rotor R becomes great. If the core stator angle .theta.A is made great, since the magnetic flux of the rotor R passes through the wide field poles .phi.1A and .phi.1B, the dentention torque is decreased in proportion with decrease in the magnetic flux density and the drive torque becomes great because the magnetic poles of the field poles .phi.2A and .phi.2B are close to the magnetic poles of the rotor R. Due to such characteristics, on using the 3-position rotational actuator, determination is made in terms of making greater account of either the detention torque or the drive torque so as to design an apparatus with an optimal core stator angle .theta.A.
However, for obtaining the most preferred 3-position rotational actuator generation of a detention torque and drive torque sufficiently suitable for various applications is required without resulting in an increase in size, weight or cost of the actuator due to use of a high magnetic force permanent magnet for the rotor, increase in the turning number of the field pole, and so on.
The first object of the present invention is to provide a 3-position rotational actuator which is excellent in both the detention torque and the drive torque irrespective of size and weight by bringing out the maximum of action of the rotor and field pole with a magnetic force and a turning number equal to those of conventional actuators.
Furthermore, the 3-position rotational actuator is constructed as follows for simpler manufacturing.
First, a 4-pole, field pole is covered by a bowl-like yoke so as to arrange a field circuit and a rotor, having an output shaft at its center position, to be rotatably supported at a center portion of the field pole. With such an arrangement of an electric section, in order to enclose the field pole and rotor, a base member having at its center portion a through-hole for penetration of the output shaft is attached the yoke to making up the 3-position rotational actuator. In addition, rotation limiting members are arranged respectively by respectively providing projections on the base member and the rotor so that the rotation of the rotor is stopped by coming into contact therewith.
However, such an arrangement causes the base member to become complex in configuration. It is necessary to provide the projections being the rotation limiting members for limiting the rotation of the rotor, to form at the center portion the through-hole for penetrating the output shaft, and to perform some machining for heightening the positioning accuracy thereof with respect to the yoke and further to have, for example, connectors for introducing lead wires to supply field current to the field pole encased therein. In order to obtain such a complex configuration, to reduce impact noises generated when the projections provided on the rotor and the base members as the rotation limiting members come into contact with each other, and to reduce the magnetic influence to a driven body as much as possible on installation of the 3-position rotational actuator, it is preferred to form the base member using a resin.
The mechanical strength of the formed resin is lower compared with that of the other pieces such as the yoke which make up the 3-position rotational actuator, and therefore the following arrangement is made when the 3-position rotational actuator is attached to a driven body. FIG. 10 is a perspective drawing showing how a bracket C of a driven body is attached to end portions of the longer axial directions of a substantially elliptical 3-position rotational actuator by means of two bolts BA, BB, FIG. 10A is a perspective view of the external appearance thereof and FIG. 10B is an enlarged cross-sectional view taken along a line I--I. As shown in the Figures, a flange Yb is formed at an open portion of yoke Ya formed by the press-machining of an iron plate. The flange Yb is integrally secured to the upper surface of a resin-made base member B by means of a number of rivets R. When the 3-position rotational actuator is attached to the bracket C of the drive body, the base member and the bracket C are fixedly secured to each other by means of the bolts BA, BB. A compressing force e to the bolts BA, BB is then applied to the base member B and therefore iron-made collars D are inserted under pressure into the insertion holes of the bolts BA, BB or integrally formed therewith in order to prevent the base member B from being broken.
Thus, although the 3-position rotational actuator itself is simple in structure and can be manufactured at a low cost, parts are required to increase the mechanical intensity of the accompanying portion to be attached to the driven body, resulting in increase in the number of the manufacturing steps and in cost.
Therefore, the second object of the present invention is to provide an excellent 3-position rotational actuator capable of simplfying the arrangement including the accompanying portion and being easily manufactured at a lower cost.
Furthermore, in a 3-position rotational actuator adapted to obtain three stable points with four poles, the rotation of the rotor R is compulsorily stopped by the rotation limiting portion to obtain two stable positions (positions A and C in FIG. 9). That is, whenever the 3-position rotational actuator is controlled to take the two positions, first and second limiting members of the rotation limiting portion are collided with each other. This causes problems such as impact noises due to collision of both the limiting members and mechanical deterioration of both the limiting members.
One possible countermeasure to remove these problems is that the rotational force of the rotor R is lowered to decrease the impact force between both the limiting members. However, most of objects using the 3-position rotational actuator require a great drive torque and a high responsiveness, and therefore this countermeasure is not useful in practice.
In addition, as a countermeasure to resolve the above-mentioned problems without lowering the drive torque and the responsiveness of the 3-position rotational actuator, there is a method in which the first and second limiting members are formed with a sufficiently elastic material. However, this method causes both the limiting members to be greatly bent due to their elastic characteristics when coming into contact with each other and therefore difficulty is encountered to accurately determine the the two stable positions of the rotor R, resulting in the lack of the basic performance necessary for the 3-position rotational actuator.
Therefore, the third object of the present invention is to provide a 3-position rotational actuator which has a great drive torque and a high responsiveness, allows accurate determination of the respective stable positions, has little operation noises and is excellent in durability.